Low energy ions found in outer space may provide answers to many questions in the fields of physics and astronomy. Gathering the information these particles hold, however, sometimes requires spacecraft equipped with sensitive instruments to detect these ions and extract information about them. To achieve this, these particle detection instruments must measure various parameters about the particles which would include their energy, atomic mass, and direction of travel. In order to develop and calibrate these instruments, low energy ion sources capable of simulating ions found in outer space are required.
A low energy ion source is described in NASA Technical Memorandum 82559, dated September 1983, by A. P. Biddle, J. M. Reynolds, W. L. Chisholm, Jr., and R. D. Hunt. This ion source is constructed having an elongated housing wherein one end is closed except that this end includes an orifice through which a gas to be ionized is introduced, and the opposite end is generally open, and through it ionized particles are exhausted. An electron emitting filament is positioned near the end having the gas orifice. Next, serially, there is located an elongated, positively biased (with respect to the filament), electrically conductive cylinder constructed of a material having a high magnetic permeability. This cylinder includes grids at each end, and the cylinder and grids together are positively biased to form an anode. This anode draws electrons from the filament and accelerates them, and ionization impacts are made on the gas within this cylinder, and thus there is in effect created an ionization chamber within the anode structure. A single coil is wound around the cylinder from end to end of the cylinder, creating a magnetic flux within the ionization chamber, with the result that ionization impacts and resulting ionized particles generally respond to this dynamic movement. The ionized particles are drawn out of the ionization chamber by a negatively biased extractor grid and then passed by a small, negatively charged, hairpin-shaped filament which is employed to limit divergence of the ion beam leaving the ionization chamber.
Despite the employment of the neutralizing filament, it has been found that ion beam divergence remains substantial. In order to tolerate this divergence and to effect some collimation of the ions, fairly long drift tubes must be employed. This in turn results in a large portion of the ions being lost, which in turn results in low beam currents insufficient to determine the dynamic range and saturation points of some instruments. Further, a large proportion of the electrons emitted by the filament are lost because they are directed to the wall of the housing without undergoing an ionizing collision. This makes it necessary to operate the filament at a higher power level to overcome these losses. As a result, relatively large quantities of unionized gas are allowed to pass into an instrument test chamber employed with the ion source, this being due to incomplete ionization of the gas. Still further, the disclosed source failed to provide sufficient output below 10 eV.
It is an object of this invention to provide an improved system for testing atomic particle detection instruments employing an improved low energy ion source and which generates a beam of ions having low divergence, is more efficient in its operation, and which provides higher beam currents in the lowest ranges of operation.